High dynamic range (HDR) displays are displays that can display imagery with very high contrast, very deep blacks and very bright whites. Such types of displays can show HDR imagery by using non-uniform backlighting. In particular, one can adjust the intensity of the backlighting on different areas of the screen based on the input image. These displays have received much attention in the recent years as an alternative format for digital imaging.
The traditional Low Dynamic Range (LDR) image format was designed for displays compliant with ITU-R Recommendation BT 709 (a.k.a. Rec. 709), where only two orders of magnitude of dynamic range can be achieved. However, real world scenes have a much higher dynamic range which are around ten orders of magnitude in daytime. The human visual system (HVS) is capable of perceiving 5 orders of magnitude.
Because most display devices have a limited dynamic range, HDR scenes shown on Low Dynamic Range (LDR) display devices usually turn out to be either saturated (corresponding to the concept of “overexposure” in photography) or extremely dark (corresponding to “underexposure”). Either case is undesired as numerous details can be lost. Therefore, a process called tone mapping or tone reproduction is needed to let the HDR contents be displayable on traditional display devices.
Tone mapping for HDR video has drawn much attention in academia as well as in industry. However, compared with the tone mapping of still images, relatively very little effort has been put on HDR video tone mapping. Particularly, tone mapping for HDR images has been studied in recent years in computer graphics as well as in image and video processing. Roughly speaking, tone mapping methods can be classified into two primary categories: global tone mapping and local tone mapping.
Although local tone mapping using a gradient domain method to video has added a smoothness constraint term along the motion direction which helps to reduce temporal brightness fluctuation, the control of the image appearance is limited as this mapping method tends to generate images with artificial look and seems to have a limited number of ways to control the image appearance.
One publication (Interactive Local Adjustment of Tonal Values by Dani Lischinski, Zeev Farbman, Matt Uyttendaele, Richard Szeliski in ACM Transactions on Graphics, 25(3) (Proc. ACM SIGGRAPH 2006), July 2006) provided a scribble based interactive tone mapping approach for HDR images. This method shows flexibility in terms of changing the appearance of image. However, the extension of such a method to HDR video is very difficult because it is not practical to draw scribble marks on each frame. Tracking scribbles along temporal axis is also not feasible if there exists complex motion.
As such, a need exists to develop a method for HDR video tone mapping that incorporates user interactivity to improve the tone mapping quality, but yet does not require the user to directly mark each frame in a video sequence to achieve high quality tone mapping.